Apparatus and method for self adjusting downlink signal communication

ABSTRACT

An apparatus for sending signals downhole during drilling operations uses a main pump to pump mud from a mud pit at a substantially constant flow rate. The bulk of the pumped mud goes downhole to maintain adequate circulation for the drill bit. A bypass pipe is provided with a shut-off valve that is controlled by an electronic controller. By pulsing the opening and closing of the shut-off valve, the volumetric flow downhole is pulsed. The pulse amplitude and duration can be controlled. These pulses in the flow rate are detected by a suitable downhole device such as a flow rate measurement device, a pressure detector or a turbine. The initial “wake-up” pulse is made long enough so that the detection device downhole is always able to detect it. Subsequent to this wake-up pulse, adjustments are made to the pulse duration, in steps of about 2 seconds, for the smallest pulse period that is detectable downhole. In addition, the amplitude of the pulses is also controlled by regulating the maximum flow through the shut-off valve. The pulses are modulated by a binary sequence of numbers corresponding to the data to be transmitted.

FIELD OF THE INVENTION

The invention relates to the control of downhole drilling equipment by amud pulse telemetry system, and particularly to an automatic adjustmentof the pulse amplitude and duration to account for the attenuation ofsignals at increased depth.

BACKGROUND OF THE INVENTION

Well bores or boreholes are drilled using a drilling assembly (alsoreferred to as the “bottom hole assembly” or “BHA”) carrying a drillbitat its bottom hole end. The BHA includes a variety of sensors to gatherinformation about the wellbore and subsurface formations along withassociated processing circuits and microprocessors. Data and signals aretransmitted from the surface to control the operation of devices in theBHA. Such devices include motors, hydraulic devices, etc. A number ofsignal transmission methods have been used to send signals from thesurface to a receiver in the BHA. In one such method, an acoustic signalcarried by the mud or by the drillstring is used. Electromagneticsignals carried by the drillstring have also been used to transmitinformation downhole. However, these methods are difficult to use inmeasurement-while-drilling (“MWD”) operations because of the necessityof maintaining an adequate mud flow for drilling operations and of thenoise associated with the mud flow and with the rotating drillstring. Acommon method of communicating the signals downhole is via drillingfluid pressure pulses (“mud poles”) generated by altering the rate offlow of the drilling mud used in drilling operations.

This is fraught with problems because of the wear and tear on the mudpumps from constant starting and stopping. A major accompanying problemis that the mud pulses attenuate and disperse as they propagate throughthe drilling mud. This dispersion is unavoidable and is caused byvarious mechanisms, including viscous dissipation in the drilling mud aswell as frictional energy loss at the borehole walls. This problem isexacerbated with increasing depth of the wellbore. When a square wave istransmitted through a dispersive medium, the received signal is nolonger a square wave; instead of a sharp change in amplitudecorresponding to the leading and trailing edges of the square wave, thereceived signal shows a gradual change in amplitude. In addition, thereceived signal is attenuated compared to the transmitted signal.

Because of the dispersion and attenuation of the signal, detection ofthe onset of the pulses and the determination of their duration can bedifficult. Without proper decoding of the pulses, control of thedownloadable equipment is lost. As noted above, the problem gets worseas drilling depth increases due to increased attenuation and dispersion.The ability to detect pulses determines the bandwidth of the mud pulsetelemetry link. Prior art techniques have relied on an ad hoc method ofdealing with the problem: the pulse duration is increased bypredetermined increments as the drilling depth increases. As an example,a pulse duration of 8 seconds is used at shallow depths, of 12 secondsat intermediate depths and of 16 seconds at large depths. This is aninefficient procedure for as it does not allow for maximum datatransmission based on the available bandwidth of the data channel.Furthermore, the limited choice of available pulse duration means thatif the predetermined discrete values are inadequate, the entiredrillstring has to be brought to the surface to adjust the downhole toolfor another date rate.

It is desirable to have a method and an apparatus for adjusting thepulse telemetry that automatically adjusts for the attenuation of thesignal by adjusting the data rate. The method should preferably make useof the full available bandwidth for signal transmission. It should alsonot require retrieval of the downhole equipment to modify the datatransmission rate. The present invention provides a downhole telemetrysystem that automatically adjusts the data rate as a function of thedeterioration of the transmitted signal during drilling of the wellbore.

SUMMARY OF THE INVENTION

The present invention is a self adjusting communication linkincorporating a mud-pulse telemetry system for controlling downholedevices. A main pump operating at a substantially constant flow ratepumps mud from a source thereof, such as a mud pit. The bulk of thepumped mud goes downhole to maintain adequate circulation through thewellbore. A bypass conduit or path is provided with a fluid flow controldevice that is controlled by a control unit or circuit. The amplitudeand duration of the mud pulses are controlled by pulsing the fluidcontrol device. These mud pulses are detected downhole by a suitabledevice, such as a flow rate measurement device, a pressure detector or aturbine. The initial or “wake-up” pulse is made long enough so that thedetection device downhole is always able to detect the wake up pulse.Subsequent to the wake-up pulse, the duration of the data pulses isadjusted, as the drilling progresses, near the smallest duration that isdetectable downhole. In the preferred embodiment, the pulse durationsare incremented in one or two second increments

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a drilling rig using the presentinvention.

FIG. 2 illustrates the pulse-like pattern of a variation in volumetricflow during the transmission of signals.

FIGS. 3a-3e illustrate the various types of pulse shapes used totransmit signals downhole according to the present invention.

FIG. 4 illustrates the modulation of a square wave by a data sequence.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is best understood by reference to the FIGS. 1-3.FIG. 1 is a schematic illustration of a drilling rig 1 that has a swivelhead 3 to which is attached the drill string 2. At the bottom of thedrill string 2 is a drilling tool 4. The drilling tool 4 is conveyed inthe borehole 5 and includes a housing 6. Attached to the housing arestabilizers 7, 8 for stabilizing for reducing vibrations and anon-rotating sleeve 9 with ribs 10 that can be extended and retracted ina controlled fashion. The ribs are used to control the direction ofdrilling of the tool. The non-rotating sleeve 9 maintains asubstantially fixed orientation in the borehole, independent of therotation of the drill string 2. The drilling tool 4, together with thedrill head 12 can be caused to rotate by using the drill string 2.

FIG. 1 also shows a mud pit 13 in which there is a supply of drillingmud 14. A mud pump 15 has an inlet pipe 15a dipping into the mud and amain pipe 16 connected to the swivel head 3. A return pipe 17 connectedto the wellhead equipment 18 at the top of the borehole discharges thereturn mud from the annular space 19 between the walls of the boreholeand the drillstring 2 into the mud pit 13.

During operations, the mud pump 15 delivers mud 14 in a circuit in thedirection of arrow 20 from the mud tank, downward through the interiorof the drillstring 2, up through the annulus 19 to the drill head 12,through the return pipe 17 into the mud pit. The mud pump is driven bymotor 22 of constant output and accordingly delivers the mud at aconstant flow rate into the main pipe 16.

Connected to the main pipe 16 and discharging into the mud pit 13 is abranch pipe 23 that has a shut-off valve 24. The shut off valve 24 iscontrolled by controller 25. In the preferred embodiment, the controlleris a computer or a processor. By manipulating the shut-off valve 24, thebranch pipe 23 may be completely closed or completely opened to allow aportion of the mud to bypass the mud flow to the borehole 5 as shown bythe arrow 26. In the preferred embodiment, the shut-off valve 24 isdesigned to allow adjustment of the maximum flow through the shut-offvalve. In the preferred embodiment, this is accomplished by a using adisc valve in which two slots are moved relatively to each other tochange the effective nozzle area. The controller 25 adjusts the relativepositions of the two slots to adjust the maximum flow deviation inresponse to the measured downhole flow rate. Such disc valves are knownand are not described here.

In an alternate embodiment, the shut-off valve does not have anadjustment for maximum flow rate; instead, downstream of the shut-offvalve, there is a throttle 27, by which the maximum amount of volumetricflow change brought about by the branch pipe 23 is controlled. Thethrottle is also controlled by the controller 25.

During uninterrupted drilling operations, the flow of mud in the mainpipe 16 is substantially constant. In order to transmit data downhole,the shut-off valve 24 is actuated so as to open the branch pipe 23. Thisreduces the flow in the drillstring 2. This change is detected downholeby a suitable device on the drilling tool 4. In the preferredembodiment, this is a turbine/generator combination (not shown) drivenby the flow of mud in the drillstring. Those versed in the art would befamiliar with such a turbine/generator driven by the flow of mud.Reduction in the rate of flow of the mud would cause a reduction in therotor speed of such a turbine and hence its output voltage. Conversely,when the shut-off valve is opened, there is an increase in the rate offlow of mud downhole and a corresponding increase in the output voltageof the generator. Other devices, such as a flow rate measurement deviceor a pressure detector could be used for detection of pulses downhole.

The turbine/generator combination is the preferred downhole detectiondevice because the output of the generator is used to drive otherdownhole devices. The downhole detector sends a signal (not shown) backto the surface indicating what has been decoded. This is accomplished byusing a conventional Measurement While Drilling (MWD) telemetry system.The information to be transmitted uphole is relatively small since allthat is essential for the invention is an indication of the data rate atwhich the downhole detector is decoding. A number of known methods couldbe used for sending this signal to the surface. This includes anacoustic signal carried by either the mud flowing uphole or by thedrillstring or an electromagnetic signal carried by the drillstring.These methods would be familiar to those versed in the art.

Depending on the manner of actuation and design of the shut-off valve, apulse-like pattern can be imposed on the volumetric flow. This isillustrated in FIG. 2 by the curve 52 of the volumetric flow rate at thesurface. The steepness of the flanks 53 depends upon the way and thespeed at which the shut off valve is actuated. In order to make theunderstanding of the invention easier, for the remainder of thediscussion it is assumed that the flanks 53 are vertical, so that thecurve 52 looks like an idealized square wave superimposed on the steadyvolumetric rate of flow.

FIG. 3a shows a comparison between an idealized series of pulses asgenerated at the surface and the signal received downhole. The curve 71here is the volumetric bleed-off produced by the opening and closing ofthe bypass valve rather than the volumetric flow in the main pipe. Ithas an amplitude given by 75. After a time 72, called the “wake-up”time, the surface signal is a square wave with a period denoted by 77and an amplitude denoted by 75. Also shown in FIG. 3a is the downholesignal 73 corresponding to the transmitted surface signal 71. As can beseen, the square wave is considerably smoothed out and somewhatattenuated. Nevertheless, a periodicity associated with 75 is stilldetectable. Given proper decoding, the transmitted signal can still berecovered. The method for decoding such a signal would be familiar tothose of knowledgeable in the art.

FIG. 3b is simply a reproduction of a portion of FIG. 3a. Thetransmitted signal 91, characterized by an amplitude 93 and a period of95 is shown. FIG. 3c is similar to FIG. 3b with a transmitted signal 101having an amplitude 103 and period 105. However, the period 105 of thesignal is less than the period 95 of the signal in FIG. 3b. As long asthe downhole signal corresponding to this surface signal can be properlydecoded, it can be seen that the signal in FIG. 3c has the capability ofcarrying more information than the signal in FIG. 3b. Within a giventime interval, more bits (zeros and ones) can be accommodated. One ofthe features of the present invention is the ability to vary the timeperiod of the signal.

FIG. 3d illustrates another variation of the transmitted signal. In FIG.3d, the transmitted signal 111 has the same period 115 as the signal inFIG. 3b; however, its amplitude 113 is different. This is accomplishedby changing the maximum amount of the volumetric flow possible in thebranch pipe 23. FIG. 3e illustrates yet variation of the transmittedsignal. Here, both the amplitude 123 and the period 125 of the pulse aredifferent from the reference pulse in FIG. 3b.

As would be familiar to those versed in the art, a data message can becoded as a sequence of zeros and ones. The present invention can be usedto perform communication from the surface to the downhole equipment byencoding the sequence of square wave pulses discussed above with thesequence of zeros and ones describing the message to be transmitted.This encoding is readily performed by modulating the square wave pulsewith the sequence of zeros and ones describing the data. FIG. 4illustrates the result of modulating a square wave sequence. The topcurve 151 illustrates an unmodulated square wave sequence having aperiod 153. The data message denoted by 155 corresponds to the binarysequence 10011. The result of modulating the square wave sequence 151 bythe modulating signal 155 is the curve 157. This can be used to changethe flow rate and control the downhole equipment for a variety ofpurposes, including control signals for a directional drilling tool,signals for switching operating modes of individual components of theunderground system.

The ability to vary both the amplitude and the period of the signal isthe basis for the adaptive nature of the present invention. The changesin the period can also be encoded as part of the signal. The first bit,which is used to start the data transmission, takes on one of a discreteset of values. The initial pulse is made sufficiently wide so that thedetection device is always able to determine the beginning of thecommand signal, i.e., able to wake up. Corresponding to each of thesevalues of the initial pulse is a value of the data rate to follow. Forexample, a data rate of 8 seconds might correspond to an initial pulseof 20 seconds, a data rate of 12 seconds would correspond to an initialpulse of 25 seconds while a data rate of 16 seconds would correspond toan initial pulse of 30 seconds. These values are for illustrativepurposes only and in actual practice, many more values could be used. Inactual practice, the pulse width is maintained at the shortest timeperiod that is detectable by the downhole device. The downhole toolsends a response signal back to the surface control device indicatingthat a pulse has been detected downhole. If, within a prespecified timeinterval after initiating a pulse sequence, the surface control unitfails to receive a response signal indicating detection of the pulse,the surface control unit adjusts the parameters of the pulse to increasethe likelihood of detection. For example, the data period could beincreased in steps of, say, 2 seconds, until it is correctly detectedand an indication received at the surface. Alternatively, the amplitudeof the pulses is increased. This is done by changing the maximum flowthrough the shut-off valve 24. As noted above, this is done by adjustinga throttle in the branch pipe 23 or by using a valve, such as a discvalve, as the shut-off valve. If, however, the amplitude is already atthe maximum possible value for the apparatus, the period would beincreased. Following this, the encoded data is used to determine theflow rate of the mud.

Persons of ordinary skill in the art will appreciate that manymodifications may be made to the embodiments described herein withoutdeparting from the spirit of the present invention. Accordingly, theembodiments described herein as illustrative only and are not intendedto limit the scope of the present invention.

We claim:
 1. An apparatus for transmitting data during drillingoperations between a surface location and a downhole location in aborehole comprising: (a) a pump at the surface for pumping mud from asource thereof; (b) a conduit for transporting the mud to the borehole;(c) a by-pass coupled to the conduit for selectively diverting the flowof mud in the conduit; (d) a flow control device associated with theby-pass; (e) a controller operatively connected to the flow controldevice to control the flow through the flow control device and cause apulsed variation in the pressure of the mud in the conduit indicative ofthe data to be transmitted; and (f) a downhole detection device adaptedto detect said pulsed variation in the pressure of the mud.
 2. Theapparatus of claim 1 wherein the flow control device is a valve havingan open position and a closed position.
 3. The apparatus of claim 2further comprising a throttle associated with the by-pass, said throttleadapted to adjust the maximum rate of flow through the by-pass.
 4. Theapparatus of claim 2 wherein the valve is a disk valve with a fixed slotand a rotatable slot, the rotatable slot adapted to be moved in responseto a signal from the controller to adjust a maximum rate of flow in thevalve.
 5. The apparatus of claim 1 wherein the controller is a computer.6. The apparatus of claim 1 wherein the pulsed variation in the pressureof the mud further comprises a “wake-up” pulse having a length followedby a sequence of square wave pulses having an amplitude and a period. 7.The apparatus of claim 6 wherein the downhole detection device isfurther adapted to send a response signal indicative of detection of asquare wave pulse.
 8. The apparatus of claim 7 wherein controllerchanges said amplitude of the square wave pulses after a predeterminedtime interval upon failure to receive a response signal at the surfacelocation.
 9. The apparatus of claim 7 wherein the controller changes andamplitude of the square wave pulse and said period of the square waveafter a predetermined time interval upon failure to receive a responsesignal at the surface location.
 10. The apparatus of claim 1 wherein thedownhole detection devices comprises a flow rate measurement device. 11.The apparatus of claim 1 wherein the downhole detection devicescomprises a pressure detector.
 12. The apparatus of claim 1 wherein thedownhole detection device comprises a turbine/generator combination. 13.A method for transmitting data during drilling operations between thesurface and a downhole location in a borehole comprising: (a) pumpingmode from a source thereof through a conduit into a drill stringdisposed in the borehole; (d) using a controller to divert part of themud into a by-pass coupled to the conduit, thereby changing a rate offlow of mud in the conduit and generating mud pulses indicative of thedata to be transmitted, said mud pulses having a predetermined rate andan amplitude; and (c) detecting the mud pulses downhole and transmittingto the controller a response signal indicating detection of said pulses.14. The method of claim 13 further comprises using the controller tochange the characteristics of the mud pulses upon failure to receive aresponse signal within a predetermined interval by changing at least oneof (i) the amplitude of the mud pulses, and (ii) the predetermined rateof the mud pulses.
 15. The method of claim 13 further comprising usingthe controller to generate a “wake up” pulse, said “wake-up” pulsehaving a period, preceding the mud pulses having an amplitude and apredetermined rate.
 16. The method of claim 13 further comprising usinga flow control device in the by-pass, said flow control device beingoperated by the controller.
 17. The apparatus of claim 1 wherein thepump at the surface pumps mud at a substantially constant flow rate. 18.The method of claim 13 wherein the step of pumping mud further comprisespumping mud at a substantially constant flow rate.
 19. An apparatus fortransmitting data during drilling operations between a surface locationand a downhole location in a borehole comprising: (a) a pump at thesurface for pumping mud from a source thereof; (b) a conduit fortransporting the mud to the borehole; (c) a by-pass coupled to theconduit for selectively diverting the flow of mud in the conduit; (d) aflow control device associated with the by-pass; (e) a controlleroperatively connected to the flow control device to control the flowthrough the flow control device and cause a pulsed variation in thepressure of the mud in the conduit indicative of the data to betransmitted, said controller further adapted for altering a parameter ofsaid pulsed variation; and (f) a downhole detection device adapted todetect said pulsed variation in the pressure of the mud.
 20. Theapparatus of claim 19 wherein the flow control device comprises a valvehaving an open position and a closed position.
 21. The apparatus ofclaim 20 further comprising a throttle associated with the by-pass, saidthrottle adapted to adjust the maximum rate of flow through the by-pass.22. The apparatus of claim 20 wherein the valve comprises a disk valvewith a fixed slot and a rotatable slot, the rotatable slot adapted to bemoved in response to a signal from the controller to adjust a maximumrate of flow in the valve.
 23. The apparatus of claim 19 wherein thecontroller comprises a computer.
 24. The apparatus of claim 19 whereinthe pulsed variation in the pressure of the mud further comprises a“wake-up” pulse.
 25. The apparatus of claim 24 wherein the downholedetection device is further adapted to send a response signal indicativeof detection of a received pulse.
 26. The apparatus of claim 25 whereinsaid controller changes an amplitude of the square wave pulse after apredetermined time interval upon failure to receive said response signalat the surface location.
 27. The apparatus of claim 25 wherein thecontroller changes said amplitude of the square wave pulse and saidperiod of the squares wave after a predetermined time interval uponfailure to receive said response signal at the surface location.
 28. Theapparatus of claim 19 wherein the downhole detection devices comprises aflow rate measurement device.
 29. The apparatus of claim 19 wherein thedownhole detection devices comprises a pressure detector.
 30. Theapparatus of claim 19 wherein the downhole detection device comprises aturbine/generator combination.
 31. The apparatus of claim 19 whereinsaid altered parameter further comprises at least one of (i) anamplitude of a pulse, and, (ii) a period of a pulse.
 32. The apparatusof claim 19 wherein the pump at the surface pumps mud at a substantiallyconstant flow rate.
 33. A method for transmitting data during drillingoperations between the surface and a downhole location in a boreholecomprising: (a) pumping mud from a source thereof through a conduit intoa drill string disposed in the borehole; (b) using a controller todivert part of the mud into a by-pass coupled to the conduit, therebychanging a rate of flow of the mud in the conduit and generating mudpulses indicative of the data to be transmitted, said mud pulses havinga predetermined rate and an amplitude; and (c) detecting the mud pulsesdownhole and transmitting to the controller a response signal indicatingdetection of said pulses; and (d) using said controller for altering aparameter of said mud pulses in response to a predetermined condition.34. The method of claim 33 further comprising using the controller tochange the characteristics of the mud pulses upon failure to receive aresponse signal within a predetermined interval by changing at least oneof (i) the amplitude of the mud pulses, and (ii) the predetermined rateof the mud pulses.
 35. The method of claim 33 further comprising usingthe controller to generate a “wake up” pulse.
 36. The method of claim 35wherein said “wake-up” pulse has a period.
 37. The method of claim 33further comprising using a flow control device in the by-pass, said flowcontrol device being operated by the controller.
 38. The method of claim33 wherein pumping said mud further comprises pumping mud at asubstantially constant flow rate.
 39. The method of claim 35 whereinsaid predetermined condition comprises a failure of said controller toreceive a response to a “wake up” pulse.